In the present study, a proprietary 0.2 tonne/h Dusty Cloud Separation (DCS) technology prototype by Value Ash Technologies NV (Belgium) was used to obtain ultra-fine fly ash as a novel by-product. Two types of Class F fly ash were used: separated fly ash (FA1) with particle size < 9.3 μm and ultra-fine fly ash (FA2) < 4.6 μm as sub-fraction of FA1. Two types of cement were used: Portland cement CEM I 52.5 R HES (CEMI) and Blast Furnace Slag Cement CEM III/A 42.5 N LA (CEMIII). The results at paste- and mortar- levels showed that an increased fineness of the fly ash (FA2) contributes to better workability of the mix. For CEMI, the compressive strength of concrete with FA2 with 25% cement replacement was already equal to the reference mix at the age of 28 days. For CEMIII, the compressive strength of concrete mix with FA2 with 25% cement replacement was equal to the reference mix value at the age of 91 days. Regarding the durability, replacing cement with ultra-fine fly ash (FA2) had a positive influence on the chloride migration coefficient and ASR, and a negative influence on the carbonation resistance.